An etched copper or printed conductive circuit pattern on a polymer film base may be referred to as a flexible circuit or as flexible printed wiring board. Originally designed to replace bulky wiring harnesses, flexible circuitry is often the only solution for the miniaturization and movement needed for current, cutting-edge electronic assemblies. Thin, lightweight and ideal for complicated devices, flexible circuit design solutions range from single-sided conductive paths to complex, multilayer three-dimensional packages.
Commonly used dielectric film base materials for flexible electronic packaging include polyimide, polyester terephthalate (PET), random-fiber aramid, and polyvinyl chloride. Changes in electronic device design create the need for materials with properties or design possibilities surpassing previously attainable performance and processing capabilities. For example, a lower dielectric constant allows faster electrical signal transfer, good thermal performance facilitates cooling for a package, a higher glass transition or melting temperature improves package performance at higher temperature, and lower moisture absorption leads to signal and data processing at higher and higher frequencies.
Polyimide film is a commonly used substrate for flexible circuits that fulfill the requirements of complex, cutting-edge electronic assemblies. The film has excellent properties such as thermal stability and low dielectric constant. Liquid crystal polymer (LCP) films represent suitable materials as substrates for flexible circuits having improved high frequency performance, lower dielectric loss, and lower moisture absorption than polyimide films.
Flexible circuits may also include circuit connecting features that fall under the general term “conductive bumps” such as those present in contact sets, also referred to as interposers, which provide temporary interconnection for example between integrated circuit testers and devices under test (DUT), during test and bum in. Other structures of this type include integrated circuit probes, and compression bumped connectors involving flex circuit-to-flex circuit, or flex circuit to circuit board interconnection.